Rollup merge of #122405 - celinval:smir-new-const, r=oli-obk

Add methods to create StableMIR constant

I've been experimenting with transforming the StableMIR to instrument the code with potential UB checks.

The modified body will only be used by our analysis tool, however, constants in StableMIR must be backed by rustc constants. Thus, I'm adding a few functions to build constants, such as building string and other primitives.

One question I have is whether we should create a global allocation instead for strings.

r? ``````@oli-obk``````

Author: matthiaskrgr